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From: Michael Robinson on 10 May 2010 00:36

"Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message
news:5Yednf5fg-xt1nvWnZ2dnUVZ_rydnZ2d(a)earthlink.com...
> miso(a)sushi.com wrote:
>> On May 8, 8:01 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
>>> John Larkin wrote:
>>>> On Sat, 08 May 2010 20:19:15 -0400, Bitrex
>>>> <bit...(a)de.lete.earthlink.net> wrote:
>>>>> If you put a reference current into the collector of one half of a
>>>>> differential pair,
>>>> You've lost me already. What do you mean by putting a current into the
>>>> collector of a diff pair? Normally one stuffs a current into the
>>>> paralleled emitters.
>>>> Can you post a schematic?
>>>> John
>>> Sorry for not being more clear - in the meantime I was able to find a
>>> website that has an example of the circuit configuration I am talking
>>> about here, figure 3:
>>>
>>> http://www.uni-bonn.de/~uzs159/
>>>
>>> It appears from the text the most common way to compensate the 1/T
>>> temperature dependence of the circuit is to use a resistor with a
>>> positive temperature coefficient in the circuit that provides the
>>> control voltage. To make it work well I guess you would want to bind
>>> the differential pair to the tempco resistor with some kind of thermally
>>> conductive adhesive.
>>>
>>>
>>>
>>>> and then take an output current from the collector of
>>>>> the other transistor of a differential pair, the output current as a
>>>>> fraction of the reference current will be compensated against
>>>>> variations
>>>>> of Is. However, because of the dependence of VT on temperature Vbe
>>>>> will
>>>>> now have a positive tempco, and the output current as a fraction of
>>>>> the reference current will decrease with increasing temperature of the
>>>>> pair. I'm wondering what strategies the analog designers here have
>>>>> used
>>>>> to compensate for this second-order temperature dependence? TIA!
>>
>> Looks like frames did you a disservice. It this the page in question?
>> http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html
>>
>> If not, then right click on the link in question and open it in
>> another window to get the correct URL.
>
> Thanks, that's the correct page.

That guy is such a bad writer. He never gives you the most basic
information: what's the circuit supposed to do, what kind of input signal
goes in, just the most basic stuff that should be the first thing he tells
you. Instead he starts out from nowhere and goes into excructiating detail
about a whole bunch of stuff that has no context. It looks like he's
describing an attempt to build a transconductance amplifier with an
exponential transfer function. And there's a sentence buried deep in the
text, something about in input of one volt per octave, and he wants the
output current to double for a change of one volt in the input signal. I
haven't figured out whether the circuit he shows is fit for that purpose.
How many octaves (volts) is the input going to vary? How big of a signal
can you put into the circuit he drew? You could spend forever getting lost
in the details. Better to ask first: what's the right approach?


From: Bitrex on 10 May 2010 00:38
Jim Thompson wrote:

> That's where I'm lost. I see no redeeming social value for the
> circuit. What will be the ultimate end use? The example seems to be
> from someone with less than a full understanding of bipolar
> transistors... although maybe he's grasping at making a log
> amplifier... and attacking it cattywampus??
>
> I posted a chip circuit here some time ago, can't seem to find it now
> (if anyone remembers, tell me what filename I posted it as :-), but
> it's gain in dB's was linear with a control voltage. All temperature
> compensation on chip, no thermistors.
>
> ...Jim Thompson

Indeed, it is a form of a log amplifier. In the analog synthesizer
world (pre-MIDI) there was a standard that control voltages feeding the
modules should cause a 1 octave change in oscillator frequency, filter
cutoff frequency, etc. for each 1 volt change in input. So if you take
the output of that current source circuit and put the current into an
integrator you'll get a ramp out, the frequency of which is directly
proportional to the current input. Connect up some kind of circuit to
discharge the integrator capacitor when it hits some threshold and you
get a sawtooth wave.

To get the 1 volt per octave curve you solve the equation VT ln (2) and
you find delta Vbe is something like 17 millivolts, so there's an input
control summer that divides down the input control voltage to that level
and is adjustable with trimmer resistors. The differential structure
cancels out the first order temperature dependence of Is, but it will
still drift around with the dependence of VT on T. I've found that
apparently the way this problem was typically "solved" was a thermistor
was used in the feedback path of the control summer, but it never really
worked perfectly.

Once microprocessors started being used in analog synthesizers several
of the more expensive models had "tune" buttons where the microprocessor
would do its best to dynamically apply a control voltage that would get
the oscillators in tune. Soon synthesis went digital and they just
became computers in a fancy box. Interestingly, though, Korg recently
came out with a true analog synthesizer for the first time in decades -
though its musical usefulness might be questioned:

http://www.youtube.com/watch?v=dNNb18aFSQ4&feature=related

In any case, the circuit as I've described it above is probably about 40
years old. I thought that there must certainly be a better way to do
things now from the analog perspective?

From: Bitrex on 10 May 2010 00:44
Michael Robinson wrote:
> "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message
> news:5Yednf5fg-xt1nvWnZ2dnUVZ_rydnZ2d(a)earthlink.com...
>> miso(a)sushi.com wrote:
>>> On May 8, 8:01 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
>>>> John Larkin wrote:
>>>>> On Sat, 08 May 2010 20:19:15 -0400, Bitrex
>>>>> <bit...(a)de.lete.earthlink.net> wrote:
>>>>>> If you put a reference current into the collector of one half of a
>>>>>> differential pair,
>>>>> You've lost me already. What do you mean by putting a current into the
>>>>> collector of a diff pair? Normally one stuffs a current into the
>>>>> paralleled emitters.
>>>>> Can you post a schematic?
>>>>> John
>>>> Sorry for not being more clear - in the meantime I was able to find a
>>>> website that has an example of the circuit configuration I am talking
>>>> about here, figure 3:
>>>>
>>>> http://www.uni-bonn.de/~uzs159/
>>>>
>>>> It appears from the text the most common way to compensate the 1/T
>>>> temperature dependence of the circuit is to use a resistor with a
>>>> positive temperature coefficient in the circuit that provides the
>>>> control voltage. To make it work well I guess you would want to bind
>>>> the differential pair to the tempco resistor with some kind of thermally
>>>> conductive adhesive.
>>>>
>>>>
>>>>
>>>>> and then take an output current from the collector of
>>>>>> the other transistor of a differential pair, the output current as a
>>>>>> fraction of the reference current will be compensated against
>>>>>> variations
>>>>>> of Is. However, because of the dependence of VT on temperature Vbe
>>>>>> will
>>>>>> now have a positive tempco, and the output current as a fraction of
>>>>>> the reference current will decrease with increasing temperature of the
>>>>>> pair. I'm wondering what strategies the analog designers here have
>>>>>> used
>>>>>> to compensate for this second-order temperature dependence? TIA!
>>> Looks like frames did you a disservice. It this the page in question?
>>> http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html
>>>
>>> If not, then right click on the link in question and open it in
>>> another window to get the correct URL.
>> Thanks, that's the correct page.
>
> That guy is such a bad writer. He never gives you the most basic
> information: what's the circuit supposed to do, what kind of input signal
> goes in, just the most basic stuff that should be the first thing he tells
> you. Instead he starts out from nowhere and goes into excructiating detail
> about a whole bunch of stuff that has no context. It looks like he's
> describing an attempt to build a transconductance amplifier with an
> exponential transfer function. And there's a sentence buried deep in the
> text, something about in input of one volt per octave, and he wants the
> output current to double for a change of one volt in the input signal. I
> haven't figured out whether the circuit he shows is fit for that purpose.
> How many octaves (volts) is the input going to vary? How big of a signal
> can you put into the circuit he drew? You could spend forever getting lost
> in the details. Better to ask first: what's the right approach?
>

It does make more sense when you get to the page above through the
main page: http://www.uni-bonn.de/~uzs159/index.html, the material then
has some context!
From: miso on 10 May 2010 02:57
On May 9, 9:44 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
> Michael Robinson wrote:
> > "Bitrex" <bit...(a)de.lete.earthlink.net> wrote in message
> >news:5Yednf5fg-xt1nvWnZ2dnUVZ_rydnZ2d(a)earthlink.com...
> >> m...(a)sushi.com wrote:
> >>> On May 8, 8:01 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
> >>>> John Larkin wrote:
> >>>>> On Sat, 08 May 2010 20:19:15 -0400, Bitrex
> >>>>> <bit...(a)de.lete.earthlink.net> wrote:
> >>>>>> If you put a reference current into the collector of one half of a
> >>>>>> differential pair,
> >>>>> You've lost me already. What do you mean by putting a current into the
> >>>>> collector of a diff pair? Normally one stuffs a current into the
> >>>>> paralleled emitters.
> >>>>> Can you post a schematic?
> >>>>> John
> >>>> Sorry for not being more clear - in the meantime I was able to find a
> >>>> website that has an example of the circuit configuration I am talking
> >>>> about here, figure 3:
>
> >>>>http://www.uni-bonn.de/~uzs159/
>
> >>>> It appears from the text the most common way to compensate the 1/T
> >>>> temperature dependence of the circuit is to use a resistor with a
> >>>> positive temperature coefficient in the circuit that provides the
> >>>> control voltage.  To make it work well I guess you would want to bind
> >>>> the differential pair to the tempco resistor with some kind of thermally
> >>>> conductive adhesive.
>
> >>>>>  and then take an output current from the collector of
> >>>>>> the other transistor of a differential pair, the output current as a
> >>>>>> fraction of the reference current will be compensated against
> >>>>>> variations
> >>>>>> of Is.  However, because of the dependence of VT on temperature Vbe
> >>>>>> will
> >>>>>>  now have a positive tempco, and the output current as a fraction of
> >>>>>> the reference current will decrease with increasing temperature of the
> >>>>>> pair. I'm wondering what strategies the analog designers here have
> >>>>>> used
> >>>>>> to compensate for this second-order temperature dependence? TIA!
> >>> Looks like frames did you a disservice. It this the page in question?
> >>>http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html
>
> >>> If not, then right click on the link in question and open it in
> >>> another window to get the correct URL.
> >> Thanks, that's the correct page.
>
> > That guy is such a bad writer.  He never gives you the most basic
> > information:  what's the circuit supposed to do, what kind of input signal
> > goes in, just the most basic stuff that should be the first thing he tells
> > you.  Instead he starts out from nowhere and goes into excructiating detail
> > about a whole bunch of stuff that has no context.  It looks like he's
> > describing an attempt to build a transconductance amplifier with an
> > exponential transfer function.  And there's a sentence buried deep in the
> > text, something about in input of one volt per octave, and he wants the
> > output current to double for a change of one volt in the input signal.  I
> > haven't figured out whether the circuit he shows is fit for that purpose.
> > How many octaves (volts) is the input going to vary?  How big of a signal
> > can you put into the circuit he drew?  You could spend forever getting lost
> > in the details.  Better to ask first:  what's the right approach?
>
>    It does make more sense when you get to the page above through the
> main page:http://www.uni-bonn.de/~uzs159/index.html, the material then
> has some context!

I thinks frames messed you up again. I have no idea where you are
trying to send us. However, I don't think that website is particularly
useful. Not only is the electronics questionable, but the page refers
to figures that are not even labeled.

SSM and Curtis Electro Music used to make chips for analog
synthesizers. ADI absorbed SSM. Doug Curtis died a few years ago, and
he had already folded that company into his design service company.
THAT makes many of the same analog circuits. I would try to find
designs from those companies rather than mess with that questionable
website.

Don't get me wrong. I would like to encourage you to do the analog
design since nowadays people think engineering is programming a damn
pic. Just don't put too much faith into one website. This stuff really
isn't rocket science, but analog can be difficult to understand if it
is not explained well.
From: Jim Thompson on 10 May 2010 10:38
On Mon, 10 May 2010 00:38:00 -0400, Bitrex
<bitrex(a)de.lete.earthlink.net> wrote:

>Jim Thompson wrote:
>
>> That's where I'm lost. I see no redeeming social value for the
>> circuit. What will be the ultimate end use? The example seems to be
>> from someone with less than a full understanding of bipolar
>> transistors... although maybe he's grasping at making a log
>> amplifier... and attacking it cattywampus??
>>
>> I posted a chip circuit here some time ago, can't seem to find it now
>> (if anyone remembers, tell me what filename I posted it as :-), but
>> it's gain in dB's was linear with a control voltage. All temperature
>> compensation on chip, no thermistors.
>>
>> ...Jim Thompson
>
>Indeed, it is a form of a log amplifier. In the analog synthesizer
>world (pre-MIDI) there was a standard that control voltages feeding the
>modules should cause a 1 octave change in oscillator frequency, filter
>cutoff frequency, etc. for each 1 volt change in input. So if you take
>the output of that current source circuit and put the current into an
>integrator you'll get a ramp out, the frequency of which is directly
>proportional to the current input. Connect up some kind of circuit to
>discharge the integrator capacitor when it hits some threshold and you
>get a sawtooth wave.
>
>To get the 1 volt per octave curve you solve the equation VT ln (2) and
>you find delta Vbe is something like 17 millivolts, so there's an input
>control summer that divides down the input control voltage to that level
>and is adjustable with trimmer resistors. The differential structure
>cancels out the first order temperature dependence of Is, but it will
>still drift around with the dependence of VT on T. I've found that
>apparently the way this problem was typically "solved" was a thermistor
>was used in the feedback path of the control summer, but it never really
>worked perfectly.
>
>Once microprocessors started being used in analog synthesizers several
>of the more expensive models had "tune" buttons where the microprocessor
>would do its best to dynamically apply a control voltage that would get
>the oscillators in tune. Soon synthesis went digital and they just
>became computers in a fancy box. Interestingly, though, Korg recently
>came out with a true analog synthesizer for the first time in decades -
>though its musical usefulness might be questioned:
>
>http://www.youtube.com/watch?v=dNNb18aFSQ4&feature=related
>
>In any case, the circuit as I've described it above is probably about 40
>years old. I thought that there must certainly be a better way to do
>things now from the analog perspective?

OK, so the problem statement is "Output current doubles for each one
volt increase in input... temperature compensated" ??

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

The only thing bipartisan in this country is hypocrisy
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